Hurricane Helene strengthened into a Category 4 storm ahead of its expected landfall on Florida's northwest coast on Thursday night, as forecasters warned that the enormous system could create a "nightmare" storm surge and bring dangerous winds and rain across much of the southeastern U.S.

Helene is about 144 kilometers south of Tallahassee and moving northeast at a speed of roughly 38 kilometers per hour, according to the U.S. National Hurricane Center. Officials are warning that Helene could bring a storm surge as high as 4.5 to 6 meters along Florida's Gulf Coast, a potentially deadly scenario.

More than 600,000 homes and businesses are without power in Florida as high winds and heavy rainfall begin to batter the state, according to the tracking site poweroutage.us.

Florida Governor Ron DeSantis stated that models suggest Helene will make landfall further east than earlier forecasts, lessening the chances for a direct hit on the capital city of Tallahassee, whose metro area has a population of around 395,000.

The shift has the storm aimed squarely at the sparsely populated Big Bend area, home to fishing villages and vacation hideaways where Florida's Panhandle and peninsula meet.

The storm's wrath was felt widely, with sustained tropical storm-force winds and hurricane-force gusts along Florida's west coast. Water lapped over a road in Siesta Key near Sarasota and covered some intersections in St. Pete Beach. Lumber and other debris from a fire in Cedar Key a week ago crashed ashore in the rising water.

Beyond Florida, up to 25 centimeters of rain had fallen in the North Carolina mountains, with up to 36 centimeters more possible before the deluge ends, setting the stage for flooding that forecasters warned could be worse than anything seen in the past century.

School districts and multiple universities canceled classes. Airports in Tampa, Tallahassee and Clearwater were closed on Thursday, while cancellations were widespread elsewhere in Florida and beyond.

While Helene will likely weaken as it moves inland, damaging winds and heavy rain were expected to extend to the southern Appalachian Mountains, where landslides were possible, forecasters said. The hurricane center warned that much of the region could experience prolonged power outages and flooding. Tennessee was among the states expected to get drenched.

Areas 160 km north of the Georgia-Florida line were expected to experience hurricane conditions. The state opened its parks to evacuees and their pets, including horses. Overnight curfews were imposed in many cities and counties in south Georgia.

"This is one of the biggest storms we've ever had," said Georgia Governor Brian Kemp.